Re: [Question: Drivers/base/core.c] Why dev->init_name = NULL in device_add function ?

[Aniroop Mathur]

On Sat, Aug 2, 2014 at 4:11 AM, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> On Sat, Aug 02, 2014 at 03:54:32AM +0530, Aniroop Mathur wrote:
>> On Sat, Aug 2, 2014 at 12:53 AM, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>> > On Fri, Aug 01, 2014 at 10:43:23PM +0530, Aniroop Mathur wrote:
>> >> Dear Mr. Greg Kroah-Hartman and Linux Community,
>> >> Greetings of the day !! :)
>> >>
>> >> I am Aniroop Mathur working on Linux Kernel for last two years.
>> >> I am stuck at one point and could not find the solution over internet.
>> >> I posted on linuxquestions.org too.
>> >> So I need your help and suggestion for it.
>> >>
>> >> Can you please help in answering my query as below:
>> >>
>> >> =====================================================
>> >> In function device_add of /drivers/base/core.c file, it is mentioned:
>> >> /*
>> >> * for statically allocated devices, which should all be converted
>> >> * some day, we need to initialize the name. We prevent reading back
>> >> * the name, and force the use of dev_name()
>> >> */
>> >> if (dev->init_name) {
>> >> dev_set_name(dev, "%s", dev->init_name);
>> >> dev->init_name = NULL;
>> >> }
>> >>
>> >>
>> >> Except forcing the use of dev_name to read device name,
>> >> Is there any other reason to make init_name as NULL ?
>> >
>> > Why would you want init_name to not be NULL?
>> >
>>
>> Currently in kernel, we cannot set name of event node.
>
> What do you mean by "event node"?
>

I am referring to the device node which HAL/Application uses
as an interface to interact with kernel for read/write data operation.
like /dev/input/event0, /dev/input/event1, etc

int fd = open("/dev/input/event0", O_RDONLY);
int res = read(fd, &input_event, sizeof(input_event));

>> If dev->init_name is not set as NULL in device_add(),
>> then I can easily set name of event node in evdev_dev.c
>> file as below:
>>
>> if(dev->init_name) {
>>       sprintf(dev->init_name, "event_%s", dev->init_name);
>> }
>> error = device_add(&evdev->dev);
>>
>> And in some input device driver code, I will use like below:
>> dev->init_name = "accelerometer";
>> input_register_device(dev);
>
> What's wrong with:
>         dev_set_name(dev, "%s", "accelerometer");
>         input_register_device(dev);
>

It is good way for setting name of "Input node".
But it will not set name of "Event node".

input_register_device call sets name of two nodes
1. Input node - sys/class/input/input<x>
2. Event node - /dev/input/event<x> or sys/class/input/event<x>

Here, input<x> and event<x> are default names set without using
dev_set_name in driver.
input<x> - kobject name of struct input_dev;
event<x> - kobject name of struct evdev;

In input_register_device(dev) function call,
dev refers to struct input_dev and not struct evdev.

struct input_dev is defined in input.h and
struct evdev is defined in evdev.c (not a header file).

So driver can use struct input_dev and set its name
using  dev_set_name(dev, "%s", "accelerometer");
But struct evdev is not accessible to driver as it is defined in
evdev.c file. So driver cannot use dev_set_name for evdev.
It can only be used in evdev.c file.

So above method output will be
sys/class/input/accelerometer
/dev/input/event<x> and sys/class/input/event<x>
(event node name not set)

>> So, overall output will be
>> /dev/input/event<x> --> /dev/input/event_accelerometer
>> sys/class/input/input<x> --> sys/class/input/accelerometer
>>
>> In short, input and event node names are set just by
>> adding one line, which i found quite efficient.
>> There is other way also to set name of event node but
>> it involves using extra variable and little more code,
>> So I am looking for best solution possible. :)
>
> Only use init_name for static struct devices, for a dynamic one, jsut
> set the name like everyone else does, how is that "more" code than
> anything else?
>

Can you please elaborate what do you mean by static struct devices ?
Can I consider embedded accelerometer, proximity, gyro sensor,
touch panel in android mobile devices as static devices ?

Input subsystem is setting default name of input and event node.
This is the normal method everyone uses.

I do not want to use default name like event0, event4, input2, etc
My aim is to set name of input and event node through driver as desired.
So after discussion of other ways to set name through driver,
we can compare which way is better.
In one other method, there is a need to add extra variable in
struct input_dev and hence more memory size.

>> >> And if it is not made NULL, is there any problem or side-effect ?
>> >
>> > Yes, people would start to use it thinking it was the real name of the
>> > device, when it might not be.
>> >
>>
>> As the name itself nicely suggests, it is just a initial name.
>
> So please do not use it, someday it will go away...
>
> thanks,
>

Thanks and Regards,
Aniroop Mathur
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